WASHINGTON — No matter who wins the 2012 election, the next president will take office as the United States faces vast new opportunities in energy production and profound challenges to environmental protection.

After decades of growing dependence on imported oil, the U.S. is moving to energy self-sufficiency, thanks to greater domestic supplies of oil and natural gas and reduced demand. Coal, which once fired most American power plants, is being edged out by natural gas, renewable energy and stricter efforts to cut pollution — a trend that has touched off bitter political fights.

At the same time, climate change has gone from distant threat to palpable reality, as ice caps shrink, winters shorten and drought spreads. Climatologists and policymakers warn that unless the United States and other industrialized nations move to rein in emissions of heat-trapping greenhouse gases by 2020, most aspects of life — from the food chain to the oceans to communicable disease — could be altered, largely for the worse.

With the stakes so high, President Obama and Republican candidate Mitt Romney offer starkly divergent ideas on how to proceed. The split between the candidates on so many energy issues might be traced, in part, to a fundamental disagreement over the reality of climate change. Romney has said he is unsure of it. Obama has called it a "threat to our children's future."

Both tout an "all-of-the-above" energy strategy that would utilize everything from coal to wind, but the candidates emphasize different fuels. Romney embraces greater reliance on fossil fuels, including coal — the greatest contributors to climate change. Obama sees a future increasingly tied to renewable energy, like wind and solar.

The Obama administration's Environmental Protection Agency has implemented rules to reduce air pollution that will further crimp coal usage. Romney has vowed to repeal those rules.

"The rhetoric of 'all of the above' is the same," said Michael A. Levi, director of the energy security and climate change program at the Council on Foreign Relations. "But the vision is very different."

Obama's record over the last four years reveals an agenda most analysts expect him to stick to if he gets a second term.

He failed to secure passage of sweeping legislation to address climate change, disappointing many supporters. Nonetheless, he has taken steps to cut greenhouse gas emissions and other pollutants.

The administration also funneled federal funds to renewable energy, helping to bring more alternative sources on line and making the country the leader in clean energy investment. But it has left itself open to political fallout from failures like Solyndra, a solar equipment maker that won $535 million in federal loan guarantees and then went bankrupt.

When Romney was governor of Massachusetts, from 2003 to 2007, he articulated an agenda similar to Obama's current one. Romney's staff included prominent environmentalists who developed a state climate action plan and a regional cap-and-trade system.

In his 2010 book "No Apology: The Case for American Greatness," he wrote: "I believe that climate change is occurring — the reduction in the size of global ice caps is hard to ignore. I also believe that human activity is a contributing factor."

But none of that hints at how Romney might govern as president, because he has retreated from those positions. Romney's website makes no mention of the environment or climate change, unlike Obama's. His ideas instead echo congressional Republicans whose narrative over the last four years is that environmental regulations kill jobs.

"Romney is where the Republican Party is," said Joshua Freed, head of the clean energy program at Third Way, a centrist Washington think thank.

Romney and Obama vary markedly about the fuels the country should rely on for electricity. Romney has pushed for coal, a cheap and abundant domestic fuel, and he has accused Obama of "waging a war on coal."

Obama's campaign rejects that notion, and it has run ads trumpeting the increase in recent years of coal production and jobs in swing states like Ohio. Domestic coal usage is in fact falling, driven mostly by cheaper natural gas prices.

Behind the pro-coal rhetoric, the Obama administration has passed or is considering regulations that would further cut coal use, including reductions in smokestack emissions of carbon dioxide, mercury and other pollutants and limits on mountaintop removal, a controversial mining method.

Romney has pledged to repeal or scale back many of those initiatives, particularly the reductions in carbon dioxide and mercury emissions. "If Romney's elected, it's not so much what would happen as what wouldn't happen," Freed said. "His plan seeks to cement the status quo at a time when the energy sector is in the midst of huge upheaval."